The present invention arose out of continuing development efforts in the noted parent applications. The invention provides a lead frame pattern having a plurality of lead frames interconnected and held together by support bridges therebetween which are designed to disappear during a direct bonding operation.
Electric circuit assemblies in the field of the present invention typically include an electrically insulating thermally conductive non-metallic refractory substrate, e.g. ceramic, having electrically conductive metallic lead frames, e.g. copper, mounted on the substrate, and electric components, e.g. semiconductor chips, mounted on the lead frames. The structure is covered by an insulating housing which in turn is mounted to a heat sink such that the bottom surface of the ceramic substrate is in intimate contact with the heat sink. Examples of such circuit assemblies are given in the noted parent applications.
The copper lead frames are mounted to the ceramic in various manners, for example by a solder reflow operation, or by a direct bond operation, both of which processes are known in the art.
Direct bonding of the copper to the ceramic involves placing the copper lead frame in contact with the ceramic substrate, heating the copper lead frame and the ceramic substrate to a temperature below the melting point of the copper lead frame to form a eutectic with the lead frame which wets the lead frame and the substrate, and cooling the lead frame and the substrate, with the lead frame bonded to the substrate. The lead frame is pre-oxidized, and the heating is done in an inert atmosphere, or alternatively the heating is done in a reactive oxygen gas atmosphere. Examples of the direct bond process are given in the noted parent applications.
The present invention arose during development efforts relating to direct bond circuit assemblies and methods involving a lead frame pattern with numerous lead frames including isolated segments or inserts of small size. This type of implementation typically arises in circuit assemblies having control logic or gating circuitry having a lead frame pattern directly bonded on the substrate. The small size of the individual isolated lead frames precludes individual placement of same onto the ceramic substrate prior to the direct bonding process. Furthermore, it is not practical to efficiently oxidize each separate lead frame segment or insert. Furthermore, it is likely that such lead frames would lose alignment during the direct bond processing.
The present invention provides a simple and effective solution to the above noted problems. A lead frame pattern is provided by a plurality of lead frames interconnected and held together by support bridges therebetween which are designed to disappear during the direct bonding operation. The support bridges have a reduced thickness relative to the lead frames and are spaced above the ceramic substrate. In one embodiment, the copper lead frames have a base thickness of 0.010", and the pattern is half etched from below to a thickness of 0.005" at the support bridges to provide short strips of copper connecting adjacent lead frames. During the direct bonding process, the support bridges open, retract and disappear. Upon completion of the direct bonding process, it has been found that there is a highly satisfactory fusion of the copper lead frames to the ceramic substrate, and that there has been virtual elimination of any remnant of the copper support bridges, i.e. the support bridges have been vaporized. The reason for the effect of vaporization of the support bridges is not entirely clear. It is believed that the spacing of the support bridge above the ceramic substrate minimizes heat transfer between the support bridge and the substrate and minimizes any heat sink cooling of the support bridge by the substrate, to aid vaporization of the support bridge .